Wires such as are used in the windings of transformers and motors are often insulated by enamels applied from organic solvent, and their application can involve health, fire and air-pollution hazards. To avoid these hazards, the wire may instead be insulated by kraft paper, but this is slow and expensive and undesirably increases the bulk of the winding.
It has long been known that epoxy resin powders provide coatings having good electrical-insulating properties without polluting the atmosphere. Most such coatings are far too hard and brittle to provide electrical-insulating coatings for wires, because they tend to crack when subjected to severe stress, either upon being bent or upon expanding and contracting with changes in temperature. The stress can be especially severe when the wire has the rectangular cross-section which is usually specified for windings of the largest wire gauges. The coatings must be resistant to abrasion at the high speeds at which the wires may be wound and to chemicals such as hot transformer oils and the automotive fluids to which starter motor windings may be exposed.
Some epoxy coating powders have been specially designed to provide good toughness and flexibility while retaining good electrical-insulating properties. See for example U.S. Pat. No. 3,477,971 (Allen et al). The powder of its Example I comprises a blend of polyglycidyl ether of bisphenol A, trimellitic anhydride and stannous octoate. Coatings of this and other powders of the patent did not meet the current rigorous specifications of at least one large manufacturer of electrical distribution and power transformers.
Another epoxy powder which was specifically developed for wire coatings is disclosed in U.S. Pat. No. 4,040,993 (Elbling et al). This patent suggests that better toughness and flexibility can be obtained from a mixture of three different epoxy resins plus a pre-prepared epoxy ester and a curing agent containing trimellitic anhydride. It is believed that this powder also did not meet the aforementioned specifications.